1. Field of the Invention
The invention relates to a rotor for an electric machine.
2. Description of the Related Art
Rotors of this type often have a rotor carrier with a disk-like support portion extending in the radial direction and a cylindrical axial support portion, on which a sheet pack consisting of lamellar electrical sheets is attached. The sheet pack can carry a coil winding or a plurality of permanent magnets, or it may not have any additional elements at all, and functions as the rotor of a reluctance machine while guiding the magnetic flux inside the rotor. The sheet pack is usually attached to the cylindrical axial support portion of the rotor carrier by a process such as pressing, adhesive bonding, caulking, or welding, as described in, for example, DE 102 26 572 A1.
During the operation of the electric machine, the rotor is subjected to extreme forces; in particular, the centrifugal forces acting on a cup-shaped rotor supported axially on one side can cause the open end to expand. In the case of rotors having a large diameter and/or rotors which operate at high revolutions per minute, at least some of the individual lamellar sheets can be spread apart or become laterally displaced or, in the extreme case, the entire sheet pack can break apart. Various types of mechanical vibration modes of the rotor are also associated with this behavior. Taken alone or in conjunction with the effects of the associated electromagnetic coupling between the rotor and the stator, these vibrations are accompanied by clearly noticeable, unpleasant running noises such as shrill whistling tones. The disadvantageous effects, however, are not limited to the electric machine alone but can also involve elements which are in working connection with the rotor, such as the crankshaft and its bearings in the case of a vehicle drive system consisting of an internal combustion engine and an electric motor, in which the rotor is connected non-rotatably to the crankshaft. In this case, the vibrations of the rotor of the electric machine can be superimposed with the vibrations of the crankshaft, which occur as a result of the periodic combustion process, in such a way that the two units start to vibrate in resonance with each other. The units can thus be prematurely damaged and can therefore fail a significant period before the end of their anticipated service life.
To counteract this problem, which has been known for many years, the common practice is to make the overall rotor very heavy, especially when it is cup-shaped, or at least to provide reinforcing elements in certain areas, preferably in the area of the open end of the cup, to prevent the sheet pack from spreading radially apart. A collar, for example, can be provided at the open end to act as a reinforcing element. These measures, all of which are familiar to the person of ordinary skill in the art, solve the problem by increasing the strength of the components and by shifting the occurring resonance frequencies as far as possible outside the nominal speed range. However, at the same time the mass of the rotor and its moment of inertia are considerably increased, which is generally undesirable for the application mentioned above.